Pixelmator Pro has permanently docked palletes docked left and right (which float above the image). The layers pallette is the one on the left and is resizable. It looks like you've inadvertently grabbed the side of it and made it huge. Just grab the edge of it with your mouse and make it smaller. You'll be able to see more of your image.

If you want to see still more, both layers and tools can be hidden (as can the rest of the interface). Just have a look in the View menu.